How to get an iPhone 3G/3GS TomTom car kit to work with an iPhone 4

Of niche interest, I know, but: if you’ve got the TomTom car kit for an iPhone 3G or 3GS, you’ll know that it doesn’t work properly with the iPhone 4’s flat back. If you don’t have, or if you’ve lost, the official adapter, sticking a £1 coin behind the bottom of the phone works perfectly.